Method of removing core from diecasting

ABSTRACT

A die casting method for producing aluminum die castings each having a longitudinally extending bore which is effective to prevent the aluminum die castings from shrinking to an extent that the desired dimensions of each casting will not be achieved. To this end, an annealing process is introduced to force the casting together with a core member, which defines the bore and which is a part of the casting dies, but which is separate therefrom, to be annealed at a predetermined temperature for a predetermined time so that internal stresses previously set up in the casting can be relieved. This annealing process is carried out immediately upon solidification of the molten metal during the casting.

United States Patent 11 1 Mino et a1.

1111 3,825,055 1451 July 23, 1974 METHOD OF REMOVING CORE FROM DIECASTING [73] Assignee: Toyo Kogyo Co., Ltd., Hiroshima-ken, Japan 22 Filed: 0111.2,1972

[21] Appl. No.: 294,299

[52] US. Cl. .Q. 164/132 8/1921 Morse 164/132 7/1931 Hourigan 11/1957 Parlanti Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel Attorney, Agent,0r Firm-Wenderoth, Lind & Ponack [5 7] ABSTRACT A die casting method for producing aluminum die castings each having a longitudinally extending bore which is effective to prevent the aluminum die castings from shrinking to an extent that the desired dimensions of each casting will not be achieved. To-this end, an annealing process is introduced to force the casting together with a core member, which defines the bore and which is a part of the casting dies, but which is separate therefrom, to be annealed at a predetermined temperature for a predetermined time so that internal stresses previously set up in the casting can be relieved. This annealing process is carried out immediately upon solidification of the molten metal during the casting.

3 Claims, 2 Drawing Figures METHOD OF REMOVING CORE FROM DIECASTING The present invention relates to a method for producing aluminum castings'which require precise dimensions.

More particularly, the present invention pertains to a method for producing aluminum die castings each having a longitudinally extending bore of a relatively large size, which method is effective to prevent the aluminum die castings from shrinking to an extent that the desired dimensions of each casting will not be achieved.

It is generally recognized that a die casting method, as compared with the othercasting methods, has an advantage in that a casting can be produced which precisely meets predetermined dimensional requirements. However, in the production of a die casting having a longitudinally extending bore of a relatively large size, which bore must be precisely formed to permit the die casting to be utilized as a final product substantially, without subjecting it to a subsequent machining process, this advantage of the die casting method over the other casting methods is not present for the reasons as hereinafter described:

I Partly because a pair of dies defining a cavity into which molten metal is poured to form a die casting tend to cool relatively rapidly and partly because the cooling speed of the molten metal poured into said cavity is also high, the internal stress in the body of the die casting as cast increases as said molten metal solidifies, which ultimately leads to a shrinkage deformation of the resultant die casting, said shrinkage deformation varying in degree among a plurality of die castings which are produced by a single die casting machine.

2 In view of the fact that it is impossible to predict during the casting operation where on the resultant die casting such shrinkage deformation takes place, no measures can be taken to avoid the possible shrinkage deformation before it actually takes place. Accordingly, a subsequent machining process is required, during which some of the die castings tend to become defective unless care is taken in'machining each of the die castings.

3 The problem of shrinkage deformation happens not infrequently in the production of die castings made of metallic alloys which are susceptible to shrinkage as said metallic alloy's cool to room temperature.

These inconveniences "are often encountered in the production of, for example, a cylinder block for an internal combustion engine or a rotor housing for a rotary piston engine, both of which are usually made of aluminum alloy.

Accordingly, an essential object of the present invention is to provide an improved method for producing a die casting which satisfies precise dimensional requirements, thus substantially eliminating the abovementioned inconveniences.

Another object of the present invention is to provide the improved method of the abovementioned character wherein, immediately after a die casting has been formed on a core which defines the bore of the resultant casting and which is situated within the cavity defined by a pair of casting dies, that is, before the die casting is cooled to room temperature, the die casting is annealed at a predetermined temperature for a predetermined time while formed on said core thereby to remove the internal stress produced within the body of the resultant die casting.

A further object of the present invention is to provide the improved method of the abovementioned character which is effective to prevent the resultant die casting from shrinking to an extent that the desired dimentions of said casting will not be achieved.

According to the present invention, as will be clearly understood from the following description thereofian essential feature resides in the provision of an annealing process to reduce or substantially eliminate the internal stress within the body of the resultant casting as the molten metal of the casting solidifies. This annealing process is preferably carried out in a furnace, heated to a temperature within the range of to 400C, for at least 10 or more minutes for the reasons diately after the molten metal of the casting has completely solidified 'within said cavity. As has been pointed out, the problem of shrinkage deformation happens not infrequently in the production of die castings made of metallic alloys which are susceptible to shrinkage as said metallic alloys cool to room temperature. Accordingly, the annealing process follows immediately after the previous process in order to relieve the internal stress within the body of the resultant casting before the resultant casting cools to room temperature.

These and other objects and features of the present invention will become apparent from the following description taken in conjunction with a preferred embodiment thereof with reference to the accompanying drawings, in which;

FIG. 1 is a longitudinal sectional view of a casting die assembly which can be used to'produce a typical die casting by the method of the present invention, and

FIG. 2 is a cross sectional view taken along the line IIII in FIG. 1.

Before the description of the present invention proceeds, it is to be noted that, for facilitating abetter understanding of the present invention, the method described with reference to making a rotor housing for a rotary piston engine. The method according to they present invention is, however, equally applicable to the production of other structural elements of metallic material' having at least one hollow portion which have precise dimensional requirements. More specifically, the method of the present invention can be effectively and efficiently used in the production of a hollow die casting of a varying thickness wherein shrinkage will otherwise take place unevenly in the circumferential direction .or the production of a die casting having a longitudinally extending bore of non circular shape.

Referring now to the drawings, a die casting, for example, a die castalumimurn rotor housing, which is wn s ca t as nt ma ed y 019? be. pmsl djn a die assembly which includes a pair of fixed and movable dies 11 and 12, and a separate core member 13 having a longitudinally extending hole 13a. The core member 13 has both end portions positioned in respective sockets 14 and 15 which, in cooperation with respect to each other, define a pair of inward projections 11a andl2a projecting into the longitudinally extending hole 13a from the both ends thereby accurately locating the core member 13 in such a manner that the outer periphery of said core member 13 is uniformly spaced from the contour of each of the recesses 16 and 17 respectively formed in the fixed and movable dies 11 and 12 so as to define a casting cavity, said casting cavity being shown as accommodating thereinthe die cast aluminum rotor housing 10. In contrast to the usual die casting techniques, this core member 13 does not need to be supported by an extra arrangement, since it is supported by merely fitting the core member 13 into either of the sockets 14 or 15 with a projection 11a or 12a in the recess-l6 or 17 being positioned in the longitudinally extending hole 13a of the core member 13. The casting cavity formed by the recesses 16 and 17in the respective dies 11 and 12 receives molten metal, for example, molten alloy of aluminum, supplied under pressure through a supply passage 18 formed in the fixed die 11', while the dies 11 and 12 are held together in any known manner. It is clear that the molten metal within the casting cavity encircles the outer periphery of the core member 13 which defines a longitudinally extending bore of the resultant die casting 10.

It is to be noted that the rotor housingfor the rotary piston engine usually-has an epitrochoidal shape and has has formed on the inner peripheral surface a pair of lobes, as at a and 10b, which protrude inwardly in the opposite directions with respect to each other. .In view of this, as clearly shown in FIG-2, the cross sec tion of at least the core member 13 must represent a particular shape so that the die castingv having an epi-' trochoidally shaped bore can be produced around the core member 13 within the casting cavity.

The longitudinally extending hole 13a of the core member 13 may have a similar shape corresponding to,

that of the cross section of the core member 13. This is particularly advantageous in that, when the resultant die casting 10 is cooled together with the core member 13 with or without the use of cooling medium passed through said hole 13a, uniform heat exchange can be achieved between the core member 13 and the resultant die casting.

As a first step in producing the die cast rotor housing, the core member 13 is fitted into the socket in either of the first and second dies 11 and 12 and the other die is subsequently pressed against the one die in any known manner such that the other socket receives therein the corresponding end of the core member 13 forming the castingcavity. No complicated centering procedure to cause both ends of the core member 13 to fit into the corresponding sockets l4 and is necessary for the reason as hereinbefore described.

Thereafter, molten metal is injected under pressure through the supply passage 18 into the casting cavity. In this case, the pressure necessary to supply the molten metal into the casting cavity must be sufficient to permit said molten metal within the casting cavity to uniformly encircle the outer periphery of the core member 13 in a reasonable periodof time. After the injection of the molten metal into the casting cavity is within the casting cavity solidifies, the resultant casting can assume a substantially'permanent set restrained by the core member 13.

Upon solidification of the molten metal at which condition it is formed into the die casting 10, the die casting 10 togetherwith the core member 13 is removed from the casting cavityin any suitable manner and readily placed in an annealing furnace arranged in the vicinity of the die assembly. In the annealing furnace, the casting 10 is annealed at a temperature within the range of from 150 to 400C. for at least 10 or more minutes to relieve internal stresses previously set up in the die casting during the casting process, where the casting 10 is made of aluminum alloy. If the annealing is carried out at a temperature lower than the abovementioned critical range, no advantage is obtained by annealing and no castings which meet the desireddimensions will be obtained. On the other hand, if the annealing is carried out at a temperature higher than the above mentioned critical range, blisters will be formed on the casting when the latter is still in a molten state and, therefore the casting will become defective.

The annealing time depends on the size or the thickness of the dies casting to'be formed. However, at least 10 minutesor more is necessary. A series of experiments has showed that, if a die cast rotorhousing of a size smaller to that described in the following example was annealed for three hours-at a temperature of C, no advantage was obtained by annealing and variation of the dimensions of the resultant casting was similar to that occuring in the conventinal method whereas, if the same annealed for 15-minutes at a temperature of 450C, blister was formed.

After the prescribed annealing time has elapsed, the die casting 10 together with the core member l3 is removed from-the annealing furnace and then readily separated from the core member 13 by any suitable method, for example, by the application of hydraulic pressure for pressing either the die casting 10 or the core member 13 so as to move it away from the other. Alternatively, in addition to the application of the hydraulic pressure, the core member 13 may be quickly cooled after having been removed from the furnace, thereby to cause radial separation thereof from the die casting by the effect of differential expansion of these parts. In this case, cooling medium such as water must be passed through the hole 13a of the core member 13.

Although the die casting 10 after having been separated from the core member 13 in the manner as hereinabove described seems to undergo shrinkage before it completely cools to room temperature, this is of no great significance. This is because such a shrinkage takes place uniformly in the circumferential direction of the resultant die casting, and the magnitude thereof can be taken into consideration prior to the casting operation and, therefore, very accurate die castings can be obtained if a core member 13 is employed which has a slightly greater size than the bore of the casting to be produced with due consideration for the shrinkage which will take place after the resultant casting is separated from the core member 13 and before it is permanently set. I

By way of example, the method of thepresent invention was utilized to produce a rotor housing made of aluminum alloy specified as AC4D according to the Japanese Industrial Standard, the substantial equivalent of which is Alcoa 355 and SAE 322. At that time, the rotor housing together with the core member 13 was annealed for 30 minutes at a temperature of 250C, after they had been removed from the casting cavity of the die assembly shown in FIGS. 1 and 2. For eomparision, a similar rotor housing was cast in accordance with the conventional method wherein no annealing process is provided and the die assembly was of the type having a core member integrally formed with either of a pair of fixed and movable dies.

The results are tabulated belowi It is to be noted that the terms "major distance and minor distance therein used mean the distance of the vertical line as viewed from FIG. 2 which extends in one point to the opposite point on the inner peripheral surface of the rotor housing through the center of the bore of said rotor housing and the distance of the horizontal line as viewed from FIG. 2 which extends from the lobe a to the opposite lobe 10b through the same center, respectively. Furthermore, it is to be noted that the desired values of the major and minor distances of the rotor housing were 238.80 and 178.40 millimeters, respectively.

In practice, the critical range of annealing temperature is from 200 to 300C if the material for the resultant casting is aluminum alloy, in which case the annealing time must be more than 30 minutes.

From the foregoing description, it has now become clear that the method according to the present invention is effective to produce die castings, particularly, die cast aluminum castings, which accurately and precisely meet the dimensional requirements.

Although the present invention has been fully described as applied to the production of the rotor housing for a rotary piston engine, it is to be noted that various changes and modifications will be apparent to having a longitudinally extending boreis to be produced by the method herein disclosed, the method of the present invention is equally applicable where the core member is provided on its outer peripheral surface partially or wholly with a reinforcement member, a sleeve member or a sprayed metal layer which is intended to be transferred to the inner peripheral surface of the resultant casting for various purposes known to those skilled in the art. Furthermore, in the case where the method of the present invention is to be utilized in the production of a casting having a bore of relatively complicated construction, the core member may be separableone-Accordingiy, such changes and modifications should be, unless otherwise departing from the true injected metal in said casting cavity whererby said injected metal shrinks around said core member, removing the solidified casting together with said core member from said casting cavity, annealing said casting on said core member by placing said casting together with said core member, before they are cooled to ambient temperature, inan annealing furnace heated to 'a temperature within the range of from C. to 400C., said annealing being carried out for at least 10 or more minutes within said annealing furnace, thereby to relieve internal stresses previously set up in said casting,

separating said casting from said core member before it completely cools to room temperature. and completely cooling said casting to enable said casting to assume a permanent set.

METHOD OF THE INVENTION DIMENSIONS OF PRODUCED ERRORS WITH RESPECT TO ROTOR HOUSINGS (m.m.) DESIRED DIMENSIONS (mm) DESIRED DIMENSIONS DIMENSIONS MAXIMUM OF OF AVERAGE A, A, MINIMUM ROTOR HOUSING CORE (A (A I MEMBER (m.m.) (m.m.)

Major distance 238.80 239.55" 238.95 238.55 238.75 +0.l5 0.25 Minor distance 178.40 179.17 I78.57 I78.I7 178.37 +0.l7 0.23

CONVENTIONAL METHOD DIMENSIONS OF PRODUCE ERRORS WITI-I RESPECT TO ROTOR HOUSINGS (m.m.) DESIRED DIMENSIQNS (mm) DESIRED DIM ENSIONS DIMENSIONS MAXIMUM OF OF AVERAGE B, B, MINIMUM ROTOR HOUSING CORE (B1) (B2) MEMBER (m.m.) (m.m.)

Major distance 238.80 239.55" 238.77 237.93 238.35 0.03 0.87 Minor distance 178.40 I79.I7* I78.36 177.42 177.89 0.04 0.98

that the of the core (8,) and 8,, and (5,) and 8,, respectively, correspond to each other.

correspond to the major and minor distances of ni produced rotor housing. respectively. (A J and A., (A,) and A 2. The method as claimed in claim 1, wherein said 3. The method as claimed in claim 1, wherein said separating comprises application of hydraulic pressure separating comprises rapidly cooling said core member to either said casting or said core member so as to move thereby to cause radial separation thereof from said said casting and saidcore member in the opposite dicasting by the effect of differential expansion. rections until they are separated from each other. i 

1. A method of producing a die casting having a longitudinally extending bore and which is made of aluminum alloy, which method comprises the steps of injecting molten aluminum alloy into a casting cavity having therein a separate core member which defines the bore of the die casting, said molten alloy being injected at a pressure sufficient to cauSe said molten alloy to uniformly encircle the outer peripheral surface of said core member within said casting cavity, solidifying the injected metal in said casting cavity whererby said injected metal shrinks around said core member, removing the solidified casting together with said core member from said casting cavity, annealing said casting on said core member by placing said casting together with said core member, before they are cooled to ambient temperature, in an annealing furnace heated to a temperature within the range of from 150*C. to 400*C., said annealing being carried out for at least 10 or more minutes within said annealing furnace, thereby to relieve internal stresses previously set up in said casting, separating said casting from said core member before it completely cools to room temperature, and completely cooling said casting to enable said casting to assume a permanent set.
 2. The method as claimed in claim 1, wherein said separating comprises application of hydraulic pressure to either said casting or said core member so as to move said casting and said core member in the opposite directions until they are separated from each other.
 3. The method as claimed in claim 1, wherein said separating comprises rapidly cooling said core member thereby to cause radial separation thereof from said casting by the effect of differential expansion. 